In general, this invention relates to water heaters and other heat transfer systems. More specifically, the invention relates to a heat exchanger and methods for transfer of heat to fluids. More particularly, the present invention relates to a heat exchanger configured for optimizing a percentage of heat exchange surface within an identified range of heat flux, or rate of heat energy transfer through a given surface, on a boiling curve.
Heat transfer fluids are heated to desired working temperatures for transfer of process heat in a wide range of water heater systems and other types such as boilers, central heaters, closed loop systems, residential and commercial water heaters, and water processing systems as an example.
Prior art heating apparatus including a central heat flame and a surrounding heater tube array is disclosed in U.S. Pat. Nos. 4,793,800, 4,723,513, 4,473,034, 4,444,155 and 4,338,888. U.S. Pat. No. 4,679,528 discloses a heating boiler having a central heating flame and a surrounding coil heater tube. The products of combustion pass radially through the heater tube coil, and are discharged through an axially extending vent.
Heat transfer fluids are typically heated as they flow through heater or heating tubes of a heat exchanger. In such cases, the liquid flow within the heater tube is characterized by a temperature profile that includes a relatively higher film temperature at radially outward regions in the heater tube and a relatively lower bulk fluid temperature at radially inward regions adjacent the central core region of the tube. This variance in temperature provides difficulties in maintaining a design operating temperature substantially at or near the upper thermal limit of the heat transfer fluid.
Therefore, there is a desire to improve the configuration of the heat exchangers to provide an optimal percentage of heat exchange surface within an identified range of heat flux, thus maximizing the efficiency of heat flux. In addition, there is a desire that the heat exchanger be compact since it will often comprise an ancillary device or a component of a more comprehensive apparatus.